Hardware Load-Balancing Options


Hardware load balancing, similar to software balancing with ColdFusion MX 7 Load Balancing, manages traffic within a Web cluster according to a specified load-balancing algorithm (such as round robin, or least connections). However, unlike ColdFusion MX 7 Load Balancing, hardware-based load-balancing devices sit in front of the Web cluster, meaning that all traffic destined for the Web cluster must pass through the load-balancing device.

Suppose you are configuring a Cisco LocalDirector to load-balance the www.mycompany.com domain, which contains three Web servers and a database server. You would configure the load balancer with the IP address that corresponds, DNS-wise, to www.mycompany.com. This address is called the virtual Web server address. On the LocalDirector, you would also configure the addresses of the three Web servers behind the LocalDirector and a load-balancing algorithm, such as least connections. LocalDirector would assign users to the server with the least load. Figure 3.12 illustrates the basic network configuration of the load balancer.

Figure 3.12. Cisco LocalDirector contains the virtual web server www.mycompany.com directing traffic to three clustered servers.


LocalDirector

Cisco LocalDirector can be set up to maintain network address translation (NAT). This feature ensures that Web servers in a cluster are not directly accessible by a public IP address, thus ensuring security. In this model, LocalDirector maintains the IP address that corresponds to www.mycompany.com, but LocalDirector has another interface on a switch that is shared by the three Web servers. LocalDirector acts as the gateway for Web servers in a cluster.

NOTE

An advantage to hardware-based load balancing is that users never know that the site is behind a load balancer (servers are not required to have their own individual DNS names). This is especially useful when dealing with bookmarks. On a site balanced with ColdFusion MX 7 Load Balancing, users can easily bookmark www1.mycompany.com because that is the address displayed in their browser. This can cause difficulty if www1.mycompany.com has problems or is permanently taken offline.


Load-Balancing Algorithms

Adding hardware load balancing gives you flexibility in traffic management, but the load-balancing method or algorithm you choose will have impact on the efficiency of your site. The wide array of load balancers available vary slightly in their operations, but a few basic methods are described here as a guideline:

  • Round Robin. Similar to round robin DNS, this load-balancing method assigns each server connections in the alternation fashion (the load balancer starts with Server 1 and assigns each user to the next server in order, and then starts again with Server 1). Unlike round robin DNS, no traffic is directed to servers that have failed; these servers are automatically removed form the cluster until they have recovered. The basic round robin method does not distribute traffic according to the number of server connections.

  • Round Robin with Least Connections. This method works in the same way as round robin, but the load balancer monitors the number of connections each server has. As new users connect to the site, the load balancer sends the users to the server that has the fewest connections, even if that server is not next in the round robin order.

  • Ratio. This type of load balancing distributes traffic among servers based on a pre determined connection ratio. The ratio is set by the administrator and can be based on forecasted load for each server in the cluster. You might configure your cluster so that two servers handle five users each, to one user for the third server. This approach would allow the third server to handle back-end processing or secure-transaction processing.

  • Priority. Similar to the ratio method, this method configures servers with a specific priority. Users are sent first to servers with a higher priority.

  • Fastest. Traffic is sent to the server with the fastest response.

NOTE

The ratio and priority methods are useful when all servers in a cluster are not of equal performance capability.


Big IP

Another widely used series of load-balancing devices is F5's BigIP series. These devices are different from LocalDirectorrather than running Cisco's proprietary operating system, they run OpenBSD Unix, which has many security features inherent in its design. BigIP load balancers can be configured through a Web interface running SSL or by connecting to the BigIP server through secure shell (SSH). Because the BigIP is running OpenBSD, it can act as a load balancer, a packet filtering firewall, and a masquerading firewall. However, implementing these features has an impact on the efficiency of the devices.

If you are using F5's BigIP series, additional load-balancing algorithms called Observed and Predictive are available. The load balancer analyzes performance trends of the clustered servers over time. Traffic is distributed based on trend data collected.



Advanced Macromedia ColdFusion MX 7 Application Development
Advanced Macromedia ColdFusion MX 7 Application Development
ISBN: 0321292693
EAN: 2147483647
Year: 2006
Pages: 240
Authors: Ben Forta, et al

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